1. Field of the Invention
This invention relates to inverter equipment which converts a direct current (D.C.) into a sine wave alternating current (A.C.). More particularly, this invention is applicable to a control circuit for an inverter which controls the on-off switching of semi-conductor switch elements with pulse width modulated signals.
2. Prior Art
FIG. 5 (prior art) is a block diagram to show the structure of a main circuit of a known inverter. A converter circuit 2 includes semiconductor switch elements 3.sub.1 to 3.sub.4 which are opened or closed in response to control signals for converting a DC current from a power source 1 to a filter 4. In this converter circuit 2, during a period when switch elements 3.sub.1 and 3.sub.4 are open and the switch elements 3.sub.2 and 3.sub.3 are closed, a voltage of level E is fed from power source 1 to a load 5 through the filter 4. During the period when the switch elements 3.sub.1 and 3.sub.4 are closed and the switch elements 3.sub.2 and 3.sub.3 are open, a voltage of negative polarity of level E is fed to filter 4. An AC voltage in the form of pulses is obtained at the output of converter circuit 2 by opening/closing alternately the pair of switch elements 3.sub.1 and 3.sub.4 and another pair of switch elements 3.sub.2 and 3.sub.3. The AC voltage is filtered into a sinusoidal form by filter 4 and fed to a load 5. The output from filter 4 is branched out to be inputted at a control circuit 6 (shown in FIG. 6 (Prior Art)) and the output therefrom is used as the above mentioned control signal.
The AC output voltage thus obtained from converter circuit 2 includes a large amount of harmonic components in addition to the fundamental frequency sine wave AC component. If these harmonic components include harmonic components of relatively lower order such as the third or the fifth harmonics, it is difficult for filter 4 to properly filter. It must have a large capacity. However, by using a large capacity filter, the transient response characteristic of the converter equipment is adversely affected. In order to alleviate such defects, there has been proposed a method which provides various patterns in the on-off operation of the switch elements 3.sub.1 to 3.sub.4 to reduce lower order harmonic content from converter circuit 2.
FIG. 6 (Prior Art) shows a block diagram of a control circuit 6 of a prior art inverter equipment.
In this prior art circuit, the sine wave AC voltage V of the inverter is rectified by a rectifier 11, then converted to a DC voltage having less ripple via a smoother 12 and compared with the reference voltage of a DC reference voltage source 10. Smoother 12 acts as a low-pass filter. If a harmonic component is generated due to the influence of load 5 in the waveforms of the sine wave AC voltage outputted from the inverter, smoother 12 attenuates such a component and any information of the AC output voltage distortion does not transfer to the control circuit. This method is therefore defective in that the distortion of the inverter output voltage waveform which might be caused by an external non-linear load (such as load 5) cannot be compensated. The dynamic level fluctuation of inverter output voltage which might be caused due to sudden changes of load 5 is attenuated by smoother 12 in this method. It takes several cycles for the inverter output voltage to be corrected and restored to a normal level and the transient variation amplitude of the output voltage becomes inconveniently large.